The carbon present in molten iron is normally in so-called flake form, and if the metal solidifies with the carbon in such form, the cast metal has low elongation and low tensile strength, making it unsuitable for certain uses. For a number of years it has been known that flake graphite can be converted to the nodular form by the use of so-called nodulizing agents, which initially were used to treat gray iron as it flowed from the melting furnance or when it was received in the ladle from which castings were poured.
More recently, the so-called in-mold process for producing nodular cast iron was developed. In this process, the mold is provided with a separate reaction chamber which contains a nodulizing agent. Molten metal to be cast comes into contact with the nodulizing agent before it enters the mold cavity. The nodulizing agent is taken up into the molten metal at a relatively uniform rate whereby the metal is uniformly treated leading to uniformity of properties throughout the cast metal.
In the in-mold process for producing nodular iron, the nodulizing agent used commercially to the substantial exclusion of all others in a magnesium ferrosilicon alloy containing on the order of 5 to 7 percent, by weight, of magnesium, about 43 to 48 percent silicon and balance iron. In certain alloys of this type, a small amount of rare earth metal, such as cerium, has been added to neutralize the effects of so-called tramp elements, and small amounts of calcium and aluminum have been included to provide graphite nucleation resulting in higher nodule counts in the cast metal. There has also been offered for sale a nodulizing agent comprising a mechanical mixture of granular magnesium and granular ferrosilicon alloy (50% Si), in the weight ratio of about one part of the former to about 15 parts of the latter, but the portion of the market represented by this product is substantially negilible.
Both of the above-described commercial products have undesirable characteristics. Magnesium ferrosilicon (43-48% Si) alloy dissolves in the molten iron at a relatively slow rate. Since casting parameters, such as casting time, temperature of metal being cast, etc. vary widely from foundry to foundry, the obtaining of inconsistent results has been a problem. Also, with such a relatively slow dissolving nodulizer, the configuration of the reaction chamber must be such as to expose to the molten metal being cast the largest possible surface area. With such an arrangement, the nodulizer, which generally is used in particulate form, may be carried as such into the casting causing undesirable defects and a less uniform casting. Further, by reason of the relatively slow rate of dissolution of the magnesium ferrosilicon (43-48% Si), there are limitations on pour time and minimum temperature of metal being poured.
The mechanical mixture of magnesium and ferrosilicon (50% Si), in addition to suffering from the same deficiencies of the magnesium ferrosilicon alloy discussed above, can undergo particle segregation in manufacture and shipment by reason of the substantial disparity between the density of magnesium (1.7 g/cc) and 50% ferrosilicon (4.5 g/cc), resulting in erratic casting results.